A standard bulb or lamp has a normally glass envelope and a base serving both for mounting of the lamp and for supplying electricity to the filament or electrodes that make the lamp generate light. The standard lamp base, for example of the E39 type, is generally cylindrical, with a side wall formed with a screwthread and an end face provided centrally with a button contact. The side wall is typically made of sheet metal, is normally clad with copper, and forms the other contact. The button contact at the end is flush or projects slightly axially from the base.
A socket for such a lamp has a side wall formed complementarily to the lamp base so that the lamp can be screwed into it. A floor of the socket has a central contact against which the base central contact bears when the lamp is screwed in. Typically the socket central contact projects upward somewhat from the floor of the socket, similar to how the lamp central contacts projects slightly downward from the base end face. This way a good electrical connection can be formed between the lamp and the base.
Certain lamps, for instance gas-discharge lamps, must only be used in certain fixtures because the lamps are capable of bursting so that they must be safely enclosed. In order to ensure that they are only used in certain sockets, these lamps have different bases, of which the E39 type is quite popular. The E39 base is of the type described above, in that it has a central base contact that is flush with or projects upward from the floor of the socket cavity.
Some models of lamps are, however, provided with an internal liner cage that eliminates the danger from explosion. Thus such a lamp does not need to be used in a closed fixture. Thus such a lamp is provided with an EX39 base. Such a base has a screwthreaded side like a standard E39 base, but at its end there is an extension stem on the end of which the central lamp contact is provided. The socket dedicated to such lamps has a floor provided around the central contact with a spacer that will prevent an E39-type lamp base from making electrical contact.
Thus an open fixture can be equipped with an EX39 socket and only explosion-proof EX39 lamps can work in it. If an E39-type lamp is screwed into it, the central base contact of such a lamp cannot be brought into engagement with the central socket contact. On the other hand if an EX39 lamp is used in a E39 socket, it will work just fine because its projecting central contact will easily contact the central socket contact. Thus if a given location has a mix of E39 and EX39 fixtures, the owner need only stock EX39 lamps because they will work in both types.
Such an EX39-type socket is known from U.S. Pat. No. 5,874,800. This socket has in the socket floor a recess inside which the central socket contact is arranged. A ceramic collar surrounding the recess extends toward the lamp base. When an E39 lamp is screwed into the socket, the lower end of the lamp base comes to rest on the collar before the central contact provided directly on the lamp base can make electrical contact with the central socket contact. Nonetheless, when an EX39-type lamp is used, the extension provided with the central base contact projects down into the recess formed by the collar and socket floor and in the final assembly position makes electrical contact with the central socket contact. As a result of this, though an E39 lamp can fit in an EX39 socket, it will not function, but an EX39 lamp can fit in an EX39 socket and function properly. An EX39 lamp can therefore be used in an open and closed fixture, but an E39 lamp can only be used in a closed fixture.
EX39 sockets are less popular than E39 sockets, since they are more expensive due to lower numbers that are manufactured. Also, practice has shown that the collar, which is usually made of the same ceramic material as the socket, can break when an E39 lamp is screwed in forcibly. Spacing of the socket-side central contact of the E39 lamp in the EX39 socket is thus no longer guaranteed. The safety function offered by coding is no longer available.